1. Field of the Invention
The present invention relates to a mobile communication terminal for use in cellular radio communication systems.
2. Description of the Related Art
In recent years, a cellular radio communication system has spread. In the cellular radio communication system, a plurality of base stations are scattered and disposed in a service area, and these base stations form radio zones referred to as cells. Moreover, the base station is connected to a mobile communication terminal via a radio channel for each cell. In this type of system, when a power supply is turned on in the mobile communication terminal, synchronization is established between the mobile communication terminal and closest base station. Moreover, after the synchronization is established, the system shifts to a standby state.
The mobile communication terminal performs an intermittent reception operation in the standby state. The intermittent reception operation comprises: alternately setting a wake-up period and sleep period in a constant standby period, so that power consumption of the mobile communication terminal is reduced.
A preparation operation for wake-up, path search operation, and reception operation of a paging channel are successively performed in the wake-up period. In an example of a wideband code division multiple access (W-CDMA) (IMT-2000: 3GPP specification), a wake-up starting process, identification process of a reception timing, and reception of a paging indicator channel (PICH) are successively performed.
In the path search, a path search window is set based on a path timing detected in the previous wake-up period. Subsequently, a path of a pilot channel coming from the base station on standby (hereinafter referred to as an active base station) is detected in the path search window, and a reception timing is identified with respect to the detected path. The reception of the paging channel comprises: receiving PICH transmitted from the active base station following the identified reception timing; and detecting an incoming message addressed to own terminal. As a result of the reception of the PICH, when the incoming message addressed to the own terminal is detected, the reception operation is continuously performed for incoming control. On the other hand, when the incoming message addressed to the own terminal is not detected, the system shifts to the sleep period.
Additionally, in this type of mobile communication terminal, a reception frequency of a receiver needs to be matched with a transmission frequency of the base station in order to demodulate an information symbol, and an automatic frequency control function (AFC) is disposed for that. The AFC detects a phase rotation direction and rotation amount of the symbol, for example, with respect to the path of a pilot channel detected in the path search. Subsequently, the reception frequency of the receiver is corrected in a direction in which the rotation of the phase is stopped. When the AFC is performed, fluctuation of the reception frequency of the mobile communication terminal can be corrected, and thereby stability of a reception demodulation process is enhanced (e.g., see Jpn. Pat. Appln. KOKAI Publication No. 2002-217776).
However, the related-art AFC corrects the frequency based on the reception result of the path of the pilot channel detected by the path search as described above. Therefore, when the phase of the reception timing of the path coming from the active base station shifts, for example, by a temperature fluctuation of local oscillation frequency of the mobile communication terminal during the sleep period, the path for use in the AFC is not found, and the frequency cannot be corrected. Moreover, a reception quality of the path coming from the active base station is deteriorated by a change of radio propagation environment. Also when the AFC performs an erroneous operation by the influence, the path for use in the AFC cannot be found, and the frequency cannot be corrected in some case. When the frequency cannot be corrected, a probability of failure in the path search becomes remarkably high in the subsequent wake-up period.
In general, when the path search continuously fails in a predetermined number of (e.g., three) wake-up periods in the standby state in the mobile communication terminal, “out of service area” is determined. That is, the out of service area is determined, although it is not the out of service area in actual. Therefore, an opportunity in which a user cannot perform communication increases, and service is unfavorably deteriorated. Moreover, when “the out of service area” is determined, the mobile communication terminal is returned to the same initial capture operation as that at a power-on time. Therefore, an unnecessary power is consumed for the initial capture operation, the power consumption of the mobile communication terminal increases the more, and battery life is reduced.